nil vs rock bottom

nil

noun
  • A score of zero 

  • Nothing; zero. 

det
  • No, not any. 

rock bottom

noun
  • The very lowest possible level. 

  • A period of extreme mental stress, often characterized by being homeless and being rejected by all friends and family. 

How often have the words nil and rock bottom occurred in a corpus of books? (source: Google Ngram Viewer )